1. Field of the Invention
The present invention relates to a semiconductor device including a field-effect transistor and a method for manufacturing the semiconductor device.
2. Description of the Related Art
Transistors used for most flat panel displays typified by liquid crystal display devices and light-emitting display devices are formed using silicon semiconductors such as amorphous silicon, single crystal silicon, and polycrystalline silicon provided over glass substrates. Furthermore, such a transistor employing such a silicon semiconductor is used in integrated circuits (ICs) and the like.
In recent years, attention has been drawn to a technique in which, instead of a silicon semiconductor, a metal oxide exhibiting semiconductor characteristics is used in transistors. Note that in this specification, a metal oxide exhibiting semiconductor characteristics is referred to as an oxide semiconductor.
For example, a technique is disclosed in which a transistor is manufactured using zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor and the transistor is used as a switching element or the like of a pixel of a display device (see Patent Documents 1 and 2).
Patent Document 3 discloses that the water content of an oxide semiconductor film is reduced by heat treatment, whereby water and hydrogen that are impurities in the oxide semiconductor film are reduced.
One of defects included in an oxide semiconductor film is oxygen vacancy. Oxygen vacancy in an oxide semiconductor film generates electrons serving as carriers. As a result, electrical characteristic variation, typically, a negative shift of the threshold voltage occurs in a transistor including the oxide semiconductor including oxygen vacancy. Further, when the oxide semiconductor film includes oxygen vacancy, as a problem, the amount of change in electrical characteristics, typically, the threshold voltage of the transistor is increased due to a change over time or a stress test (e.g., a bias-temperature stress test, which is also referred to as a BT stress test) and the reliability is reduced.
Thus, Patent Document 4 discloses that an insulating film from which oxygen is released by heating is formed as an insulating film in contact with an oxide semiconductor film by a sputtering method and subjected to heat treatment, whereby oxygen is supplied from the insulating film to the oxide semiconductor film and oxygen vacancy in the oxide semiconductor film is compensated.